The present disclosure relates to a satellite system where Frequency Division Duplexing (FDD) is used. Different frequencies are used for the forward traffic (i.e. traffic transmitted from the satellite to the terminals) and for the return traffic (i.e. traffic transmitted from terminals to the satellite) of the RF link, and the system is characterized in that terminals cannot receive traffic while they are transmitting traffic. In order to accommodate this limitation, and at the same time make efficient use of both uplink and downlink capacity, the system must perform specialized forward link multiplexing and return link capacity assignment.
The term “satellite system(s)” referred to hereinbelow, should be understood to encompass any one or more members of the group that consists of geo-stationary satellite systems, Low Earth Orbit (“LEO”) satellite systems and Medium Earth Orbit (“MEO”) satellite systems and other types of platforms such as High-Altitude Platforms (“HAP”) which are quasi-stationary aircrafts that provide means of delivering a service to a large area while remaining in the air for long periods of time, High-altitude, long-endurance unmanned aerial vehicles (“HALE UAV”), and the like.
In a typical satellite communications network a portion of the available capacity is allocated for hub-to-satellite communications in the forward link. Similarly, a portion of the return link capacity is allocated for satellite-to-hub communications. Although these portions of the link capacity, allocated for communicating with the hub, (also referred to as an earth station, gateway or teleport), are not discussed explicitly in the following description, still, it should be noted that the methods and air interface protocols discussed in the following disclosure may as well, and typically are, implemented in such a hub, in case where the satellite serves merely as a “bent pipe”. Namely, the satellite does not process the signals it receives other than carrying out a basic filtering thereon and shifting them in frequency. Typically, for the capacity portions allocated in the uplink (namely, for transmitting hub to satellite communications and terminal to satellite communications) the allocated frequency is substantially different from the frequency allocated for carrying out downlink communications (i.e. satellite to hub communications and satellite to terminal communications), using the capacity portion allocated therefor.
One approach for scheduling the transmissions is to perform on-the-fly transmit-receive conflict resolution without imposing any limitation on the terminals by inducing a framing mechanism thereon. To do that, a scheduler must ensure that packets are only multiplexed onto the forward link at such times that they arrive at the terminal when it is not transmitting. This means, in turn, that the forward link multiplexer must maintain a separate queue for each (active) terminal and, in addition, track the propagation delay between the satellite and that very same terminal. Once every return link time slot, and for each non-empty output queue, the scheduler would use the delay information to consult the return link capacity allocation matrix in order to check whether, at the projected time of forward link packet reception, the terminal is scheduled to transmit or not. The scheduler must then serve fairly the non-blocked queues. In addition, scheduling must allow terminals certain pre-agreed short transmission windows for random-access return link transmissions. Finally, return link capacity allocation must keep a terminal's transmission duty cycle below 100% to ensure that it can send forward link traffic without excessive delay.
Transmit-receive scheduling also impacts terminal handover between beams and satellites. With the scheme described above, the scheduler must be involved in each handover in order to make sure that forward link data is correctly re-routed.